This invention relates to a light-sensitive silver halide grain suitable for higher sensitization, a method for producing the same and a light-sensitive silver halide photographic material containing the same.
In recent years, extremely high levels of photographic characteristics of silver halide emulsion have been demanded such as high sensitivity, excellent graininess, high sharpness, low fog density, high maximum density, etc. Improvement of these characteristics is attributable to how sensitivity of silver halide grains contained in the emulsion to light is enhanced. That is, if grains having high sensitivity can be obtained, it becomes possible to make finer the silver halide grains for obtaining a light-sensitive material with a desired sensitivity, whereby its image quality, fog, etc., can be improved as is well known in the art. In the prior art, the demand for high sensitization has been primarily directed to light-sensitive material for nega using silver iodotromide emulsion, but recently higher sensitization is also demanded strongly for the purpose of improving efficiency in print working for the light-sensitive material using silver chlorobromide emulsion for color paper, etc., which has been accepted as having relatively low sensitivity. Thus, under the present situation, developments of higher sensitization techniques applicable for silver halide grains having various silver halide compositions have been continued.
Responding to these demands, as a high sensitivity emulsion, an emulsion containing 0 to 15 mol % of iodine in the form of silver iodobromide has been well known. As the methods for preparing these emulsions, there have been heretofore known the methods for controlling pH condition, pAg condition such as the ammonia method, the neutralization method, the acidic method, etc., and the mixing methods such as the single jet method, the double jet method, etc.
In order to accomplish further higher sensitivity, fine graininess, high sharpness, low fog on the basis of these known techniques, more precise technical means have been developed and researches have been made in silver iodobromide emulsion about emulsions in which not only crystal habit, grain size distribution, but also the concentration distribution of iodine within individual silver halide grains are controlled.
The most orthodox method for accomplishing high photographic performances as mentioned above is to enhance quantum efficiency of silver halide grains, and the research speculating on the influence of grain size distribution by calculating theoretically quantum efficiency is reported in, for example, preliminary textbook of Tokyo Symposium concerning progress of photography in 1980, "Interactions between Light and Materials for Photographic Applications", page 91. According to this research, it is suggested effective for improvement of quantum efficiency to use a mono-dispersed emulsion having a narrowed grain size distribution. Further, in the step of chemical sensitization practiced after formation of grains, a mono-dispersed emulsion may be considered advantageous for accomplishing high sensitivity with good efficiency while maintaining low fog.
For producing industrially a mono-dispersed emulsion, control of the speed for feeding silver ions and halogen ions in an amount theoretically determined to the reaction system and sufficient stirring are requlred under strict control of pAg and pH, as described in Japanese Unexamined patent Publication No. 48521/1980. The silver halide grains produced under such conditions comprise the so called normal crystals having a shape of either cube, octahedron or tetradecahedron, which have been known to be highly sensitized.
Also, as the silver halide grains which can give further higher sensitivity, Japanese Unexamined patent Publication No. 35440/1986 and No. 83531/1986 disclose silver iodobromide grains having the (110) face and semi-(110) face, respectively. Also, in Japanese Patent Publication No. 42737/1980, there is disclosed a photographic emulsion containing rhombododecahedral silver chlorobromide grains having the (110) face as having smaller fog. Further, Japanese Patent Applications No. 20593/1986 and No. 35585/1986 disclose photographic emulsions containing silver bromide, silver iodobromide, silver chlorobromide, silver chloroiodobromide having the (nn1) face, and it is shown that the (nn1) face has the same meaning as the above mentioned semi-(110) face.
The silver halide grains having the (110) face and the (nnl) face as described above can afford considerable improvements in aspects of sensitivity or fog, but they cannot be said to be satisfactory yet as seen from the levels of performances demanded presently for light-sensitive materials, and under the present situation silver halide grains with further higher sensitivity and lower fog have been required.